1. Field of the Invention
The present invention relates to a flat panel display, and more particularly, to a driving method and device for a flat panel display, capable of enhancing picture quality by improving contrast of image during the display of the image.
2. Discussion of the Related Art
There have been actively developed flat panel displays, such as Liquid Crystal Display (LCD), Field Emission Display (FED) and Plasma Display Panel (PDP), which can display images on screens.
In a PDP, an ultraviolet ray generated by gas discharge excites a phosphor to generate a visible ray. The PDP displays images by using the visible ray. The PDP is thinner and lighter than a CRT that has been mainly used. The PDP is advantageous to an implementation of high fidelity and large-sized screen. Generally, a PDP includes of a plurality of discharge cells arranged in a matrix configuration. One discharge cell represents one pixel in a screen.
FIG. 1 illustrates a frame including eight subfields in a driving method for a conventional plasma display panel. As shown in FIG. 1, a frame includes several subfields (for example, eight subfields) the discharge numbers of which are different from each other in order to represent a gray level of an image. Each of subfields has a reset period to remove wall charge of an entire cell uniformly, an address period to form wall charge in cells of a specific position, and a sustain period to represent an image on a screen by representing the gray level according to the discharge number. The discharge number of each subfield is classified by eight bits. For example, in order to set the brightness of some specific cell to be level of 112, fifth, sixth and seventh subfields SF5, SF6 and SF7 are addressed and subsequently discharges occur at the corresponding subfields by 24, 25 and 26 times respectively. Accordingly, the sum of the discharge numbers achieves the emission brightness of level of 112. Here, the sustain period is increased at each subfield by the ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) while the reset period and the address period are the same at each subfield.
FIG. 2 illustrates structure of a conventional PDP driving device. Referring to FIG. 2, the conventional PDP driving device includes a frame memory 2 and an image processor 4, which are connected between an input line 1 and panel 6, and the image processor 4 includes a gamma correction unit 8, a gain control unit 10, an error propagation unit 12, a subfield mapping unit 14, a data sorting unit 16, an APL arithmetic unit 18 and a waveform generation unit 20.
The frame memory 2 stores an original image inputted from the input line 1 frame by frame and supplies the gamma correction unit 8 with the stored original image.
The gamma correction unit 8 performs an inverse gamma correction based on the original image inputted from the frame memory 2 and linearly transforms the ratio of the gray level of an output image to the gray level of the original image.
The gain control unit 10 converts the gray level range of the output image transformed linearly by the gamma correction unit 8 into a predetermined gray level range.
The error propagation unit 12 propagates the cell's the error components generated from the image outputted from the gain control unit 10 to neighboring cells. This allows a user to finely adjust a brightness value.
Brightness weights for subfields are in advance assigned to the subfield mapping unit 14. Accordingly, the subfield mapping unit 14 maps data to corresponding subfields according to the gray level of the original image passing through the error propagation unit 12.
The data sorting unit 16 transforms the data mapped to the subfield mapping unit 14 to be suitable for the resolution format of the PDP and supplies the transformed data to an address driving IC of the panel 6.
Meanwhile, the APL arithmetic unit 18 finds an average picture level (APL) for the output image transformed linearly by the gamma correction unit 8 and generates a signal of level N corresponding to the number of sustain pulses based on the APL. The waveform generator 20 generates a timing control signal according to the signal of level N generated by the APL arithmetic unit 18 and supplies the timing control signal to an address driving IC, a scan driving IC and a sustain driving IC of the panel 6. Here, the address driving IC, the scan driving IC and the sustain driving IC are connected to an address electrode, a scan electrode and a sustain electrode of the panel 6, respectively, which is not shown in FIG. 2.
Since the PDP configured as above displays the gray level of the original image on a screen without any process, it is impossible to obtain a clear image. Especially, in case of a moving image, an improvement in a picture quality cannot be expected. For example, according to the PDP driving device shown in FIG. 2, the minimal distribution lower gray level (MIN) and the maximal distribution lower gray level (MAX) are set to be 0 and 255, respectively. In this case, a dynamic range cannot be adjusted in response to the gray level change of the original image. Here, the dynamic range is the range between the minimal distribution lower gray level (MIN) and the maximal distribution lower gray level (MAX) of the original image or the gray level variation range.
As mentioned above, in case that the dynamic range cannot be adjusted in response to the gray level change of the original image, the original image cannot be displayed perfectly. And also, even though the picture quality is improved by adjusting the dynamic range using the minimal distribution lower gray level (MIN) and the maximal distribution lower gray level (MAX), following problems may occur.
First, colors may change too far from the original image in comparison with the original image in a signal processing for adjusting a dynamic range.
Second, as a result of the dynamic range adjustment, the original images moved to a dark portion deteriorates due to a low gray level which displays the image on a screen without any expression.
Third, as a result of the dynamic range adjustment, the original image the gray level of which is equal to or greater than the minimal distribution upper gray level (MAX) is converted into 255 gray levels so that the expression of the bright image deteriorates.
On the other hand, a gray level of an output signal of flat panel displays such as an LCD, an FED and a PDP is not shown linearly with respect to that of an input signal so that the original image is displayed differently according to inherent input/output characteristic of the flat panel displays. Today, each of the flat panel displays performs an inverse correction to correct gray level so as to be suitable for its own displaying characteristic.
FIG. 3 illustrates a gray level correction using a fixed gamma curve in a conventional CRT. The CRT nonlinearly displays an output gray level with respect to an input gray level. This is due to the inherent input/output characteristics of the CRT itself. Accordingly, image media such as broadcast systems obtain and transmit an image using a gamma curve as shown in FIG. 1 so as to linearly display an input/output gray level of the CRT. Consequently, the input/output gamma curve in the form of a straight line is made due to the inherent input/output characteristic of the CRT.
Unlike the CRT, the flat panel display shows the inherent input/output characteristic in the form of a straight line. Since the images obtained by broadcast systems are, however, transmitted in a nonlinear form, the flat panel display is forced to correct the input/output characteristic of the gray level to be linear through an inverse gamma correction.
As described above, the flat display device as well as the CRT applies the fixed gamma curve to all images across the board. If the fixed gamma curve is applied to all images across the board, contrast in all images may be deteriorated. Since the slope of the gamma curve approaches to zero especially at a low brightness gray level, an image quality may deteriorate very seriously due to round off error at the low brightness gray level.
As a result, if the same gamma curve is applied to all images, the desired contrast is not obtained and therefore image display quality is also degraded.